Freezing detection method for fuel cell

ABSTRACT

In a method for detecting the freezing of water within a fuel cell, precise detection can be performed using a phenomenon specific to the time when water starts to freeze to allow a reduction in erroneous activation. Detection at an early stage after the start of freezing is allowed, and hence measures can be taken against an output reduction before the water within the fuel cell completely freezes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a freezing detection method for a fuelcell. More particularly, the present invention relates to a freezingdetection method for detecting a change in one of an output voltage of afuel cell, an output current thereof, cell resistance, cell temperature,and cell fastening pressure to enable more prompt and reliable detectionof the start of the freezing of water within a fuel cell unit comparedwith a conventional detection method.

2. Description of the Related Art

When a fuel cell is used under a low-temperature environment, watergenerated within the fuel cell may freeze. Depending on a place wherefreezing occurs, reaction gas intake becomes difficult, which leads toproblems such that a sudden output drop gives damage to a connectiondevice, and adversely affects a member constituting the fuel cell.Accordingly, in the case of mounting the fuel cell in an automobile anda portable device which may be used under a low-temperature environment,there is required a method which enables, when water within the fuelcell starts to freeze, prompt and precise detection of freezing withinthe fuel cell before the whole fuel cell freezes.

As a method for preventing the freezing of a fuel cell, there is aconventional method which constantly monitors an outside airtemperature, and activates a heating unit when the outside airtemperature reaches a level under a freezing point. However, because afuel cell generates heat through power generation, the fuel cell doesnot freeze momentarily after reaching a level under the freezing point.In addition, because an amount of generated heat depends on thesituation of the time in which power is generated, a temperature atwhich the freezing starts sequentially varies. That is, in the method,the heating unit is activated even in a non-freezing situation, anenergy loss increases. Accordingly, an energy efficiency can beincreased if the method is improved so as to take measures afterdetecting the start of freezing. For example, as described in JapanesePatent Application Laid-Open No. 2002-313391, there is proposed a methodin which a heating unit is activated when an output voltage is low, andthe heating unit is halted when the output voltage is high. On the otherhand, Japanese Patent Application Laid-Open No. 2005-142022 proposes amethod in which a fuel cell is determined to freeze when, at an outsideair temperature under the freezing point, a cell resistance graduallydecreases before abruptly increases.

However, as the cause of changes in cell resistance and output power,various factors can be considered, such as load fluctuations, a dry-outphenomenon in which an electrolyte membrane dries, and a floodingphenomenon in which generated water clogs a supply path for a reactiongas.

That is, with the conventional technology described above, it isdifficult to distinguish phenomena other than freezing as describedabove from real freezing so that it is difficult to precisely detectfreezing. Consequently, the conventional technology had the problem thatwaste of energy may occur due to a malfunction such as the activation ofa heating unit in the event of a phenomenon other than freezing.

SUMMARY OF THE INVENTION

The present invention is directed to a method which assumes that aphenomenon specific to the time when water starts to freeze is adetection target to promptly and precisely detect the freezing of waterwithin a fuel cell.

The present invention provides a freezing detection method for a fuelcell having the following features.

The freezing detection method for a fuel cell unit for detecting a startof freezing of water within the fuel cell unit, the method includes:

a first step of determining whether or not at least one of a timedifferential value of an output measurement value of the fuel cell unitand a secondary time differential value thereof is a positivepredetermined value or more; and

a second step of determining, when at least one of the time differentialvalue of the output measurement value and/or the secondary timedifferential value thereof is the positive predetermined value in thefirst step, whether or not at least one of the time differential valueof the output measurement value and the secondary time differentialvalue thereof subsequently change to a negative value.

According to the present invention, a phenomenon specific to the timewhen the water within the fuel cell unit starts to freeze is detected,whereby a prompt and precise detection of the freezing of the fuel cellunit is enabled. As a result, there can be provided a freezing detectionmethod for a fuel cell with a less malfunction and a less energy loss.By utilizing the present invention, it is possible to provide a fuelcell in which damage to a device resulting from a sudden output drop ofthe output due to freezing can be reduced, and in which degradation dueto repeated freezing is suppressed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a fuel cell.

FIG. 2 is a schematic cross-sectional view of a fuel cell stack.

FIG. 3 is a graph representing a change in output voltage when waterwithin the fuel cell starts to freeze.

FIG. 4 is a graph representing a change in the differential value of theoutput voltage when the water within the fuel cell starts to freeze.

FIG. 5 is a graph representing a change in the secondary differentialvalue of the output voltage when the water within the fuel cell startsto freeze.

FIG. 6 is a graph representing a change in output current when the waterwithin the fuel cell starts to freeze.

FIG. 7 is a graph representing a change in cell temperature when thewater within the fuel cell starts to freeze.

FIG. 8 is a graph representing the relationship between an amount of thechange in output voltage and an amount of water within the fuel cellwhen the water within the fuel cell starts to freeze.

FIG. 9 is a graph representing a change in the output voltage of thefuel cell in the case where the freezing is detected using a freezingdetection method according to the present invention, and then a heatingunit is activated in Example 1.

FIG. 10 is a graph representing changes in output current before andafter the fuel cell stack starts to freeze in Example 2.

FIG. 11 is a graph representing a change in output current when the fuelcell stack is short-circuited after the freezing of the fuel cell stackis detected using the freezing detection method according to the presentinvention in Example 2.

FIG. 12 is a graph representing a change in output voltage when a fuelcell stack freezes in Comparative Example.

DESCRIPTION OF THE EMBODIMENTS

A freezing detection method for a fuel cell according to the presentinvention is a freezing detection method for detecting, based on achange in output measurement value, that water within a cell freezesduring power generation in an environment where an outside airtemperature is under a freezing point. The output measurement valuesmentioned herein include individual measurable parameters such as anoutput voltage, an output current, a cell resistance, a celltemperature, and a cell fastening pressure. Among those parameters, atleast one is used.

The embodiments of the present invention are described.

First, a structure of a fuel cell is described. Although a polymerelectrolyte fuel cell is used herein as an example, the presentinvention is not limited thereto, and can be appropriately used also toanother type of fuel cell. FIG. 1 is a schematic cross-sectional view ofthe fuel cell. FIG. 2 is a schematic cross-sectional view illustratingan example of a fuel cell stack.

In FIGS. 1 and 2, as each of electrolyte membranes 11 and 12, there isused, e.g., a proton conductive polymer material, specifically one of aperfluorcarbon-based ion exchange film, a non-perfluoro-based ionexchange film, and a hybrid-based ion exchange film is used.

Examples of fuel electrodes 12 and 22 and oxidizer electrodes 13 and 23include one obtained by making carbon powder carrying fine platinumparticles into a paste with the proton conductive polymer material, andforming the paste on the surfaces of the electrolyte membranes 11 and 12by screen printing. The present invention is not particularly limited tothe materials and the method described above.

In each of the fuel electrodes 12 and 22, the fuel is dissociated intoprotons and electrons under the action of a catalyst contained in eachof the fuel electrodes 12 and 22. As the fuel, a gas such as hydrogen,or a liquid such as methanol or ethanol is typically used. The protonsgenerated in each of the fuel electrodes 12 and 22 moves in theelectrolyte membranes 11 and 21 in a state where the protons arehydrated with water molecules present in the electrolyte membranes 11and 21. On the other hand, the electrons are extracted from anextraction electrode to the outside to flow in a load circuit.

In each of the oxidizer electrodes 13 and 23, the protons, theelectrons, and an oxidizer react with each other under the action of acatalyst contained in each of the oxidizer electrodes 13 and 23 togenerate water. A part of energy generated by a sequential chemicalreaction is utilized as electric energy. As the oxidizer, oxygen in anambient atmosphere is typically used. The water generated by a powergeneration reaction normally moves in the form of water vapor or liquidwater together with a flow of the oxidizer from the oxidizer electrodesto be discharged to the outside. Alternatively, the water may passthrough the electrolyte membranes 11 and 21 to be discharged from thefuel electrodes 12 and 22.

Accordingly, as more oxygen in the ambient atmosphere exists in theoxidizer 13 and 23, the oxidation of the protons is easier, and thepower generation reaction is more active. If water generated in theoxidizer electrodes 13 and 23 freezes in an environment under thefreezing point, the generated ice inhibits a supply of the oxidizer sothat the power generation reaction becomes difficult. Once the waterwithin the fuel cell starts to freeze, it is necessary to stop thefreezing of the water before the ice completely prohibits the supply ofthe oxidizer.

Gas diffusion layers 14 and 24 perform the function of smoothing theintake of the reaction gas as well as the discharge of the generatedwater. As a material for the gas diffusion layers 14 and 24, carbonpaper and carbon felt obtained by graphitizing porous sintered carbonand a thermosetting resin at a high temperature can be used.Alternatively, there can also be used a structure having a microporouslayer which is formed by applying an ink as a mixture of a fluorineresin and carbon black particles to carbon cloth, which is produced byknitting carbon fiber and subjected to a water repellent treatment usinga fluorine resin.

In each of collectors 15 and 25, a flow path along which the reactiongas flows is formed, and a metal plate obtained by plating SUS with goldand a plate obtained by molding fine carbon particles with a resin canbe used.

As sealing members 16 and 26, a gasket made of rubber such as siliconrubber or viton rubber, or various adhesives including a hot-melt typeone can be used.

An end plate 27 is for fixing a stack on both sides thereof with afixing member (not shown) to hold the structure of the stack, whileapplying a cell fastening pressure to each of cells constituting thestack. For the end plate 27, any material can be used as long as thematerial meets the purpose thereof, and a metal such as SUS and a resinmaterial having a high strength can be suitably used.

FIG. 3 illustrates an example of a change in output voltage when waterwithin the fuel cell unit starts to freeze when a load is applied to thefuel cell with a constant current.

The inventors of the present invention found that, when the water withinthe fuel cell starts to freeze, the output voltage of the fuel celltemporarily rises rapidly, and then abruptly drops, as illustrated inFIG. 3. Conceivably, this may occur under the influence of volumeexpansion and heat of solidification resulting from a water behaviorsuch that, even at a temperature under the freezing point, the watergenerated within the fuel cell remains existing in a liquid state, i.e.,in a so-called supercooled state till immediately before freezing, andstarts to freeze when triggered by any factor. That is, the output ofthe fuel cell temporarily rises due to a reduction in the contactresistance of the fuel cell unit caused by the volume expansion when thewater in the supercooled state freezes, or due to a rise in thetemperature of the fuel cell unit caused by the emission of the heat ofsolidification. Thereafter, as the area occupied by the water whichchanges into the ice increases within the fuel cell, the intake of theoxidizer gas such as air by the oxidizer electrode becomes graduallymore difficult, so that the output voltage drops. The phenomenon is achange in output voltage which is specific to the time when freezingstarts.

FIG. 4 is a graph representing a value obtained by differentiating theoutput voltage of FIG. 3 with respect to time. When the water within thefuel cell starts to freeze, the time differential value of the outputvoltage temporarily increases, and then abruptly shifts to the negativeside. Accordingly, when the time differential value of the outputvoltage changes to a value not less than a positive predetermined value,it is determined whether or not the time differential value subsequentlychanges to a negative value to enable the determination of whether ornot freezing starts.

FIG. 5 is a graph representing a secondary time differential value ofthe output voltage obtained by further differentiating the timedifferential value of FIG. 4 with respect to time. When the water withinthe fuel cell starts to freeze, the secondary time differential value ofthe output voltage initially increases, then decreases, and increasesagain in the negative range to return to a value around zero.Accordingly, similarly to the time differential value, when thesecondary differential value of the output voltage changes to a valuenot less than a positive predetermined value, it is also determinedwhether or not the time differential value subsequently changes to anegative value to enable the determination of whether or not freezingstarts, in the same manner as with the time differential value.

The start of freezing can also be detected by individually using one ofthe above-mentioned time differential value of the output voltage andthe above-mentioned secondary time differential value thereof alone.However, for more precise detection, it can also be determined thatfreezing starts only when both of the time differential value and thesecondary time differential value satisfy criteria for determination.

When precision is further pursued, there may be added a third step ofdetermining whether or not the time differential value of the outputvoltage and the secondary time differential value thereof change tonegative values and then reach values not more than negativepredetermined values.

As the time for determining the start of freezing, different times indifferent steps may be used selectively and appropriately depending onthe structure of a target fuel cell system. For example, when the sizeof the fuel cell system is not limited, all the steps may be usedappropriately in order to increase precision. When the fuel cell systemis to be reduced in size, the minimum required steps may be performedappropriately in order to reduce a measurement system.

It is necessary to distinguish a change in output voltage indicating thestart of freezing from a change resulting from noise. As a result ofconducting intensive study on fuel cells with various sizes and shapes,the inventors of the present invention have concluded that, as long as achange of not less than ±5 mV/sec and a change of not less than ±5mV/sec² are used as respective criteria for determination for the timedifferential value and the secondary time differential value, the changeindicating the start of freezing can be distinguished from noise inalmost all cases.

FIG. 6 illustrates an example of a change in output current when thewater within the fuel cell unit starts to freeze when a load is appliedto the fuel cell with a constant voltage.

Similarly to the output voltage, the output current also temporarilyincreases, and then decreases when water in the supercooled statefreezes. In each of the time differential value and the secondary timedifferential value also, the same change as occurring in output voltageoccurs when freezing starts. Therefore, the change in output current canalso be used to detect freezing. As a result of conducting sequentialstudy, the inventors of the present invention concluded that, as long asa change of not less than ±5 mA/sec and a change of not less than ±5mA/sec² are used as respective criteria for determination for the timedifferential value of the output current and the secondary timedifferential value thereof, a change in output current indicating thestart of freezing can be distinguished from noise in almost all cases.

FIG. 7 illustrates an example of a change in cell temperature when thewater within the fuel cell unit starts to freeze when the fuel cell isdriven in an environment under the freezing point.

When the water within the fuel cell freezes, heat of solidification isemitted as a result of the freezing of the water in the supercooledstate. As a result, similarly to the output voltage and the outputcurrent, as illustrated in FIG. 7, the cell temperature also temporarilyrises, and then drops because the cell is cooled by the outside airtemperature. Regarding the time differential value of the celltemperature and the secondary time differential value thereof also, thesame changes as occurring in output voltage and output current occurwhen freezing starts. Therefore, the cell temperature can also bemeasured and used to detect freezing in the same manner as the outputvoltage and the output current are used. The inventors of the presentinvention concluded that, as long as a change of not less than ±0.2°C./sec and a change of not less than ±0.2° C./sec² are used asrespective criteria for determination for the time different value ofthe cell temperature and the secondary time differential value thereof,a change in cell temperature indicating the start of freezing can bedistinguished from noise in almost all cases. The measurement of thecell temperature can be performed at a place as close as possible to theelectrolyte membrane. As a method for measuring the cell temperature, atypical method using a thermocouple and a thermistor can be listed.

As described above, when the water within the fuel cell unit freezes,volume expansion or the emission of heat of solidification occurs, andthe cell resistance lowers under the influence of a reduction in contactresistance caused by the volume expansion and an increase in the protonconductivity of the electrolyte membrane caused by a temperature rise.Due to the reduction in cell resistance, the above-mentioned changes inoutput voltage and output current appear. A change in cell resistancecaused by the start of the freezing of the fuel cell unit is larger inthe amount of change per unit time than other changes including a changein resistance due to the wet condition of the electrolyte membrane and achange in cell resistance due to degradation. Accordingly, when the cellresistance is measured to be determined that the measured valuedecreases by an amount not less than a predetermined amount of change,and that the time differential value further reaches a predeterminedvalue, it can be determined that the freezing of the cell starts.Further, in order to enhance precision, the determination of a positiveor negative time differential value and a secondary time differentialvalue can also be used for the cell resistance in the same manner asused for the other parameters. Among changes in cell resistance, inorder to distinguish a change resulting from freezing from a changeresulting from noise, a change of 5 mΩ, a change of not less than ±5mΩ/sec, and a change of not less than ±5 mΩ/sec² may be usedappropriately as respective criteria for determination for thepredetermined amount of change of the cell resistance, the timedifferential value thereof, and the secondary time differential valuethereof. As a method for measuring the cell resistance, a generallyknown method can be used. For example, a current interruption method, analternating current impedance method, and a step method, can be listed.

When the water within the fuel cell freezes, a cell fastening pressureincreases due to the volume expansion of the water. In general, thefastening pressure of the fuel cell unit may be reduced by the looseningof a screw used for fastening, but does not increase as long as a forcefrom the outside is not applied. Therefore, when the cell fasteningpressure increases by an amount not less than a predetermined amount ofchange, it can be determined that the water within the fuel cell startsto freeze. As a result of repeatedly making detailed examination on thevalue of the predetermined amount of change, the inventors of thepresent invention found that, as long as a change of 10 kPa is used as acriterion for determination, a change resulting from freezing can bedistinguished from noise in almost all cases regardless of the size andshape of the fuel cell unit. After the water within the fuel cellcompletely freezes, the cell fastening pressure is held constant tillthe ice melts to return to water. As a method for measuring thefastening pressure, a generally known method can be used. For example, amethod which uses a piezoelectric element or a strain gauge can belisted.

As a result of further making detailed examination on those detectionmethods, the inventors of the present invention found that there arecorrelations between the amount of the water within the fuel cell andthe range of the change of each parameter. FIG. 8 is a graphillustrating the relationship between the amount of the water within thefuel cell and the amount of change of the output voltage. From FIG. 8,it is understood that, as the amount of the water within the fuel cellunit represented by the abscissa axis increases, the amount of change ofthe voltage at the start of freezing increases.

Therefore, by calculating the amount of the water within the fuel cellfrom a cumulative amount of generated power, the amount of change ofeach parameter can be estimated. By setting a predetermined amount ofchange of each parameter based on the estimated value, and examiningwhether or not the change of not less than the predetermined amount ofchange has occurred, a more precise determination can be made, and falseactivation can be prevented. The method of recognizing the amount of thewater within the fuel cell is not limited to the method of calculatingthe amount of generated water from the amount of generated power. Theamount of the water within the fuel cell unit may also be actuallymeasured using a humidity sensor.

Thus far, two methods which are the method of setting the predeterminedamount of change in advance as described above, and the method ofchanging the predetermined amount of change according to the amount ofthe water within the fuel cell have been listed. However, whether eitherone of the two methods is to be selected or the two methods are to beused in combination is determined appropriately according to thespecifications of the target fuel cell. That is, when the fuel cell isused as a power supply for a portable device, reduction in size isrequired, and a system for determining the amount of change of the waterwithin the fuel cell may not be mounted desirably. In such a case, themethod of setting the predetermined amount of change in advance can beused. On the other hand, when more precise detection of freezing isrequired for cold district specifications, the method of changing thepredetermined amount of change according to the amount of the waterwithin the fuel cell can be used.

In order to enhance the preciseness of freezing detection, it is moreeffective to use respective changes in a plurality of parameters incombination.

Hereinbelow, the examples of the present invention will be described.

EXAMPLE 1

First, a membrane electrode assembly (MEA) obtained by bonding 2 cmsquares of platinum black as the catalyst layers 12 and 13 to both sidesof a Nafion (registered trademark of Du Pont Kabushiki Kaisha) as theelectrolyte 11 by a hot press was prepared. Then, both sides of the MEAwere sandwiched between carbon cloth pieces each as the gas diffusionlayer 14, and further sandwiched the MEA from the outside between thecollectors 15 obtained by plating SUS with gold and formed with thereaction gas flow paths so as to be fastened. In this manner, a fuelcell as illustrated in FIG. 1 was produced.

The fuel cell thus produced was placed in an environment tester, andheld in a constant atmosphere at −15° C. for a period of one hour ormore. Then, power generation was started with a constant current of 50mA/cm². After the power generation was continued for a while, a rise involtage was observed at a certain time, as illustrated in FIG. 9. Thechange in voltage indicates that freezing had started within the fuelcell. According to the criterion for determining the start of freezingin the present invention, at the time when the differential value of theoutput voltage changed to a value not more than −5 mV/sec (second step)after reaching the value not less than 5 mV/sec (first step), the fuelcell was heated using a heating unit. After heating was performed, theoutput voltage that had temporarily dropped was gradually recovered toenable the power generation to be continued, as shown in FIG. 9.

EXAMPLE 2

Using the same fuel cell as used in Example 1, the same test asconducted in Example 1 was conducted except that freezing was detectedusing the secondary differential value of the output voltage instead ofthe differential value thereof. That is, at the time when the secondarydifferential value of the output voltage reached a value not more than−5 mV/sec² (second step) after reaching the value not less than 5mV/sec² (first step), the fuel cell was heated using a heating unit. Inthis case also, after heating was performed, the output voltage that hadtemporarily dropped was gradually recovered to enable the powergeneration to be continued.

EXAMPLE 3

In Example 3, the fuel cells produced in Example 1 were stacked in a3-cell series configuration to produce a fuel cell stack as shown inFIG. 2. Then, in the same manner as in Example 1, the fuel cell stackwas placed in an environment tester, and allowed to stand in anatmosphere at −15° C. for a period of one hour or more. Thereafter,power generation was started with a constant voltage of 2.4 V. After thepower generation was performed for a given period of time, a change incurrent indicating that freezing had started within the stack asillustrated in FIG. 10 appeared. At the time when the differential valueof the output current changed to a value not more than −5 mA/sec afterreaching the value not less than 5 mA/sec and when the secondarydifferential value of the output current reached a value not more than−5 mA/sec² after reaching the value not less than 5 mA/sec², the fuelcell was connected to a short circuit, and short-circuited. FIG. 11represents the transition of the current value after the fuel cell wasshort-circuited. As illustrated in FIG. 11, by short-circuiting the fuelcell, power generation could be continued without completely freezingthe inside of the fuel cell.

COMPARATIVE EXAMPLE

In Comparative Example, a fuel cell stack having the same structure asthat of the fuel cell stack used in Example 3 was used. After the fuelcell stack was held in a constant test environment at −15° C. for aperiod of one hour or more in the same manner as in Example 3, powergeneration was performed with a constant current of 100 mA. In the samemanner as in Example 1, a rise in voltage indicating that freezing hasstarted within the cell was observed. In Comparative Example, even whenthe change in voltage was observed, a procedure for continuing the powergeneration was not performed instantly. Instead, after the outputvoltage had dropped to 0 V as illustrated in FIG. 12, the fuel cell wasconnected to a short circuit, and short-circuited in an attempt tocontinue power generation. However, after the output voltage had droppedto 0 V, the power generation could not be continued even when the fuelcell was short-circuited because the inside of the fuel cell hadcompletely frozen.

From those results, it is understood that, by using the freezingdetection method according to the present invention, a procedure ofpromptly recovering the output can be performed before the fuel cellfreezes to fall into an inoperable state, and that power generation canbe thereby continued.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-046238, filed Feb. 27, 2008, which is hereby incorporated byreference in its entirety.

1. A freezing detection method for a fuel cell unit for detecting astart of freezing of water within the fuel cell unit, comprising: afirst step of determining whether or not at least one of a timedifferential value of an output measurement value of the fuel cell unitand a secondary time differential value thereof is a positivepredetermined value or more; and a second step of determining, when atleast one of the time differential value of the output measurement valueand the secondary time differential value thereof is the positivepredetermined value in the first step, whether or not at least one ofthe time differential value of the output measurement value and thesecondary time differential value thereof subsequently change to anegative value.
 2. The freezing detection method for a fuel cell unitaccording to claim 1, further comprising, after the second step, a thirdstep of determining whether or not at least one of the time differentialvalue of the output measurement value and the secondary timedifferential value thereof is a negative predetermined value or less. 3.The freezing detection method for a fuel cell unit according to claim 1,wherein the output measurement value is at least any one of a voltage, acurrent, and a cell temperature.
 4. The freezing detection method for afuel cell unit according to claim 3, wherein the output measurementvalue in the first step is a voltage, and the positive predeterminedvalue of the time differential value and the positive predeterminedvalue of the secondary time differential value are 5 mV/sec and 5mV/sec², respectively.
 5. The freezing detection method for a fuel cellunit according to claim 2, wherein the negative predetermined value ofthe time differential value and the negative predetermined value of thesecondary time differential value in the third step are −5 mV/sec and −5mV/sec², respectively.
 6. The freezing detection method for a fuel cellunit according to claim 3, wherein the output measurement value in thefirst step is a current, and the positive predetermined value of thetime differential value and the positive predetermined value of thesecondary time differential value are 5 mA/sec and 5 mA/sec²,respectively.
 7. The freezing detection method for a fuel cell unitaccording to claim 2, wherein the negative predetermined value of thetime differential value and the negative predetermined value of thesecondary time differential value in the third step are −5 mA/sec and −5mA/sec², respectively.
 8. The freezing detection method for a fuel cellunit according to claim 3, wherein the output measurement value in thefirst step is a cell temperature, and the positive predetermined valueof the time differential value and the positive predetermined value ofthe secondary time differential value are 0.2° C./sec and 0.2° C./sec²,respectively.
 9. The freezing detection method for a fuel cell unitaccording to claim 2, wherein the negative predetermined value of thetime differential value and the negative predetermined value of thesecondary time differential value in the third step are −0.2° C./sec and−0.2° C./sec², respectively.
 10. The freezing detection method for afuel cell unit according to claim 1, further comprising, determiningwhether or not the output measurement value changes by an amount notless than a predetermined amount of change which is determined accordingto an amount of the water within the fuel cell unit.
 11. A freezingdetection method for a fuel cell unit for detecting a start of freezingof water within the fuel cell unit, comprising: a first step ofdetermining whether or not a cell resistance of the fuel cell unitdecreases by an amount not less than a predetermined amount of change;and a second step of determining, when the cell resistance decreases byan amount not less than the predetermined amount of change in the firststep, whether or not at least one of a time differential value of thecell resistance and the secondary time differential value thereofreaches a negative predetermined value.
 12. The freezing detectionmethod for a fuel cell unit according to claim 11, comprising, after thesecond step, a third step of determining, when at least one of the timedifferential value of the cell resistance and/or the secondary timedifferential value thereof reaches the negative predetermined value inthe second step, whether or not at least one of the time differentialvalue of the cell resistance and the secondary time differential valuethereof changes to a positive value.
 13. The freezing detection methodfor a fuel cell unit according to claim 12, comprising, in third secondstep, determining whether or not at least one of the time differentialvalue of the cell resistance and the secondary time differential valuethereof is a positive predetermined value or more.
 14. The freezingdetection method for a fuel cell unit according to claim 11, wherein thepredetermined amount of change of the cell resistance in the first stepis determined according to an amount of the water within the fuel cellunit.
 15. The freezing detection method for a fuel cell unit accordingto claim 11, wherein the predetermined amount of change of the cellresistance in the first step is 5 mΩ, and the negative predeterminedvalue of the time differential value of the cell resistance and thenegative predetermined value of the secondary time differential valuethereof in the second step are −5 mΩ/sec and −5 mΩ/sec², respectively.16. The freezing detection method for a fuel cell unit according toclaim 13, wherein the positive predetermined value of the timedifferential value of the cell resistance and the positive predeterminedvalue of the secondary time differential value thereof in the third stepare 5 mΩ/sec and 5 mΩ/sec², respectively.
 17. A freezing detectionmethod for a fuel cell unit for detecting a start of freezing of waterwithin the fuel cell unit, comprising, determining whether or not a cellfastening pressure of the fuel cell unit increases by an amount not lessthan a predetermined amount of change.
 18. The freezing detection methodfor a fuel cell unit according to claim 17, wherein the predeterminedamount of change of the cell fastening pressure is determined accordingto an amount of the water within the fuel cell unit.
 19. The freezingdetection method for a fuel cell unit according to claim 17, wherein thepredetermined amount of change of the cell fastening pressure is 10 kPa.20. A freezing detection method for a fuel cell unit, which uses thefreezing detection methods according to claims 1 to 19 in combination.